Work continued on the DarkAero 1 prototype in June, with the first engine start successfully completed! A preliminary measurement of the full airframe empty weight was obtained, and the first session of the DarkAero Mold Making Course was held.

First Engine Start
The big milestone for June was test running the engine for the first time. The objective with this test was to confirm the functionality of the engine installation and reveal any necessary modifications as well as understand how the engine interacts with the airframe.

A number of tasks were completed over the course of the past month to prepare the engine for operation. Parts of the engine had been disassembled several times as the firewall forward arrangement for the prototype was being established. The engine and its supporting accessories were reassembled, connections were checked, and fasteners were torqued to their proper values. Additionally, several small custom pieces of hardware were fabricated and installed. The engine was primed with oil prior to the first start by turning it over using the starter motor alone and one spark plug removed from each cylinder.

The ULPower UL520iS engine used in the DarkAero 1 was supplied as a nearly complete firewall forward package. Few deviations were made from the standard configuration, so it was possible to work through the checklist established by UL Power to help ensure nothing important was overlooked prior to the first engine start. Only two minor snags were encountered while working through the checklist. A stray nut was identified that had fallen and lodged itself in the alternator housing, and a leak was caught at the fuel fitting plumbed into the discharge of the fuel pump manifold.

After the checklist was complete, the actual sequence of starting the engine was quite simple. It fired up nearly immediately and ran smoothly. As it ran, the oil pressure, fuel pressure, engine RPMs, cylinder head temperatures, and exhaust gas temperatures were monitored. It was run without the cowling, spinner, and cooling boxes installed to provide better inspection and maintenance access if anything was leaking or required adjustment. The cowling was also left off for safety reasons, as any potential fires could be more rapidly caught and extinguished. The entire airframe was secured with straps tied into several hundred pounds of sandbags to prevent it from rolling away as the brakes were not installed yet. After running for a few minutes, the engine was shut down and inspected for potential leaks and anomalies.

Leading up to starting the engine, the team was quite stoic and focused on the task at hand. However, after the test was completed, the excitement was hard to contain. The event carried much more weight than many other milestones previously achieved as the airplane suddenly felt undeniably real.

Flutter Testing and Analysis
The flutter testing and analysis has moved forward in parallel to work on the DarkAero 1 prototype. The first part of the flutter modeling, called ground vibration testing, requires measuring the natural vibration frequencies, damping, and mode shapes of the aircraft for different flight configurations. This will be accomplished by instrumenting the airframe with accelerometers in specific locations and exciting the structure with a modal hammer. During this testing, the airplane will be suspended with bungees to isolate the rigid body modes of the aircraft. The bungees will interface with the aircraft through straps that will cradle the airframe, and a chain hoist will be used to lift the assembly. Hardware for this testing was purchased, and aspects of the vibration test setup were validated. Alongside this work, a finite element aeroelastic model is being developed to refine flutter predictions.

Empty Weight Measurement
A secondary objective was accomplished while validating the hardware for ground vibration testing: a measurement of the full airframe empty weight was obtained. A load cell was added to the rigging that lifted the airframe, which allowed the empty weight of the aircraft to be measured while the airframe was suspended. An initial measurement of 638 lbs was obtained, but this value did not include all the hardware that will be installed in the prototype. The weights of omitted parts were added to the measured value to predict the full airframe empty weight of the prototype which is projected to be around 780lbs. This number includes several pieces of hardware that are specific to flight testing and will allow for faster configuration changes while testing. The empty weight of an aircraft as light as the DarkAero 1 is significantly impacted by small configuration changes. Details like the seat cushions, instruments, paint, and interior upholstery can easily sway the empty weight of the aircraft by 25 pounds in either direction, so the selection of these items must be approached with the same care as the rest of the aircraft design. Once everything in the prototype is updated to the production intent configuration, the empty weight is projected to drop to around 750lbs, which is the original target empty weight. 

Mold Making Course
The first session of the DarkAero Mold Making Course was held in June. The new course was born out of interest from students who had attended the DarkAero Aerospace Composites Course and expressed interest in developing a deeper understanding of building tooling to manufacture composite parts. The goal of the two-day course is to provide a foundation of knowledge and hands-on training so that building molds does not become a barrier in the process of manufacturing composite parts. There has been enough interest for more sessions to be scheduled. Contact courses@darkaero.com if you are interested in attending a future session of the Mold Making Course. In addition to this course, upcoming sessions of the DarkAero Aerospace Composites Course are available on July 7-8, August 4-5, and September 16-17. Seats can be reserved through our website at: darkaero.com/courses 

In The Press
DarkAero was featured in an article in the July issue of Madison Magazine! The article can be accessed through the link provided here:

DarkAero article in Madison Magazine

Looking Ahead
In July, we will be running the engine more to further understand and validate the engine installation. In parallel, we will continue wrapping up many of the small miscellaneous tasks on the prototype. Remaining projects include the flap controls, main landing gear actuation, and paint. AirVenture will be held during the last week of July, and in the past we have displayed progress on the DarkAero 1 at this event. Rather than disassembling the aircraft and trailering it to the airshow, we will continue working to complete outstanding tasks on the prototype to move as quickly as possible towards the first flight. We are filming a few new YouTube videos highlighting exciting recent progress, and these will be released in early July.